Magnetic field sensors are widely deployed in consumer and industrial instruments for applications varying from position sensing, current sensing, data storage, and magnetic compassing. There are many methods to sense magnetic fields including Hall-effect, magneto-diode, magneto-transistor, magnetoresistive-effect, magnetic tunnel junction, magneto-optical, fluxgate, search coil, and Lorentz force-effect.
The Hall effect sensor fabricated by means of CMOS technology is preferred due to its low-cost batch fabrication with CMOS technology. However, a CMOS Hall sensor features mediocre performance with marginal sensitivity for electronic compass application are often corrupted by sensor offset which is about 1000× larger than the signal generated from the earth magnetic field. The offset is resulted from fabrication gradients of the magnetic sensor doping, misalignment of fabrication process, and mechanical stress through the piezoelectric effect. In addition, the thin-film process of CMOS technology is ideal to sense the magnetic field perpendicular to the chip surface (said Z-axis) and it's challenging to construct on-chip 3-axis Hall sensor required by low-cost electronic compass applications.
Currently, two solutions to address this issue are System-in-Packaging with three single-axis Hall sensors and Hall sensors with integrated magnetic concentrator (IMC). Electronics compass constructed by either System-in-Packaging with multiple sensors and Hall sensors with IMC has large azimuth error due to sensitivity mismatch, non-orthogonality between axes, and misalignment between sensors and IMC. In general, 1% of sensitivity mismatch causes an azimuth error of 0.3 degree. The mismatch could be worse at different operation temperatures. The non-orthogonality between axes is another source of inaccuracy. For two degrees of the non-orthogonality, the maximum azimuth error is approximately two degrees. The sensitivity mismatch problem is commonly addressed by costly compensation techniques with temperature reading from an on-chip temperature sensor and the non-orthogonality is addressed by either trimming or compensation. In addition, the drawbacks for current technologies include bulky and costly for the System-in-Packaging solution and large offset and hysteresis due to the existence of magnetic material for the Hall sensors with IMC solution.
CMOS Hall sensors are constructed with an N-well layer utilizing its high mobility and are preferred for measuring out-of plane (or Z-axis) magnetic fields because of its shallow structures. The shallow structures lead to high sensitivity and easy to implement symmetric input/output terminals resulting in a high resolution, shown in FIG. 1.
A Z-axis Hall sensor comprises a Hall plate 1(a)-1(d), a protection layer (2) above the Hall plate, a substrate (3) beneath the Hall plate, and five or more terminals for electrical connections 4(a)-4(n). Two of the four terminals, 4(a)-4(c), are source terminals connected to electrical ground and an electrical supply such as voltage source or current source, respectively. The remaining two terminals 4(b)-4(d) are sensing terminals which are connected to voltage sensing circuitry.
The additional terminals, Tsub, 4(e)-4(h) are designed to provide electrical isolation between the Hall plate and CMOS substrate. The output of voltage sensing circuitry is proportional to the difference of the voltages of the sensing terminals 4(b)-4(d) and is a function of the Z-axis magnetic flux density. More than one Hall plate could be used per axis to enable more balanced bridge measurements to reduce the offset. Furthermore, dynamic offset cancellation techniques such as a spinning current method could be adapted to lower the offset further.